is seen. We also measured contrast sensitivity withEstivez and Spekreijw (1971) describe a “specrrai out rhs red field in order to replicate the conditions with kvhich the quantum compensation method” used by Est&sz and Spekreijse. TO keep the green catch of a single color mechanism can be modulated mechanism at the same mean adaptation level. the in time. while the other color mechanisms receive red field was replaced with a >eliow &Id with a a constant number of quanta and are therefore not dominant ~~,a&xgth of 570nm. the luminance of modulated.’ With this method. the\ measured the which was selected to give the same quantum catch remporal modulation sensitivity f&ions (De Lange to the green mechanism as the red field did. Since funcrions) of the human red and grcrn mechanisms, at 5-O nm the sensitivities of the rzd and green and showed that the sensitivities of these mechanisms art similar (Stiles, 1963). this means that mechanisms are indistinguishable. A striking feature the red and green mechanisms were held at the same of their results is that no low-frequency attenuation mean adaptation level in the presence of the yellow was found when stimuli were modulated sinusoidallq field. so that their sensitivities can be directly comat frequencies as low as 0.5 Hz. In this respect. their pared. The xellow field did not change the appearresults are quite different from the functions that are measured with white light (e.g. de Lange. 19331, ance of the stimulus. but only diluted the contrast. so that when the stimulus was modularsd above which eeneraliy show a marked low-frequency threshold, a hue shift was still seen. attenuation. On the basis of the data then available. ;Ilodulation sensitivity functions thar i\ere meaEst~vez and Spekreijse concluded that low-frequent! sured Lvhsn the red and green mechanisms were attenuation does not occur within individual color separately modulated in the presence of the qellow mechanisms. but probably is caused by opponent field are sho\vn by the filled circles and triangles interactions between the red and green systems when in Fig. 1. These functions show little low-frequency both are modulated by white @ht. They rejected the possibility that the high sensitivity that they attenuation. and resemble the functions reported by found at low frequencies was due to the hue shifts Esticez and Spekreijse. When both the red and green that are apparent at these frequencies. because they mechanisms are modutated simultaneously (which found neither low-frequency attenuation nor hue gives the appearance of a yellow field that Rickers shifts when they measured contrast sensitivity with in luminance). the usual low-frequency attenuation a deep red light that was modutated in iummance. occurs (crosses. Fig. 1). However. when zhe yellow We will present data that show that the detection field is replaced by the red, so that hue shifts are of low-frequency flicker can be due to hue shifts. no longer seen. the low-frequency sensitivity of the and that there is no need to postulate temporal green mechanism (open triangles) falls to the same interactions between color systems. level as that measured with luminance flicker. The Our stimuli and apparatus were identical to those high-frequency branch of the sensitivity functions is in the previous study, but hue shifts were eliminated not affected bv replacing the yellow field with the by superposing on the flickering stimuli a steady red which shows that the detection of high-frered fieid with a dominant wavelength of 677 nm and quency flicker depends only on luminance moduiaa luminance of 320cd;m’. Without this added field. tion of the preen mechanism, and not on hue shifts. when either the red or the green mechanism is slowly Because the red and yellow fields were selected modulated above threshold the stimulus appears to to stimulate the green mechanism equally. the absoalternately become reddish and greenish. (This is true lute positions of the two sensitivity functions for regardless of whether the red or the green the green mechanism can be directi! compared. This mechanism is modulated: the appearance of hue is not true for the red mechanism. for the red field depends on the relative stimuiation of the two sysdepressed its sensitivity more than the mellow field terns.) When the additional red field is introduced. did. In order to correct for this loss in sensitivity. the appearance of the field changes to a red field it is sufficient to know the relative sensitivities of that is modulated in luminance. and no hue shift the red and reen mechanisms to the red field. At these luminance levels. it is probabIe that the mechanisms that are active are similar to Stiles’ ’ FYe fiax msasured the spectral sensitivities of rhe high-intensity mechanisms, xj’ and x4’. The differred and grsen “mechanisms” using stimuli similar IO. but ence between the sensitivities of TT~’ and TC&’at kss bright CllOcd,m”~ than those used in ths present 677 nm. extrapolated from the data of Stiles (W~Zexprriment. The results resemble Stiles‘ xj and zi field secki and Stiles. 1967). is 1 log unit. and the measrnsitivit? functions. To the extent that ;t+ ressmblcs the sured sensitivities of the red mechanism v,ere sensitivity of a visual pigment. our preen mechanism may r&ot activitr in a single receptor class. but our red accordingly raised by this amount. relative to the mechanism. like xi. ma> be too broad to originat;- in a green mechanism (open circles. Fig. 11. 1.4 similar single known pigment. correction of 0.9 108 units is obraincd rf we take
. .
. .
l
l
I
0.1 100
,
0.1
,
/ , , , , /,(
,
,
/
/
, , , ,,
1
,
/
/
, , , , ,
j
100
10 Frequency I Hz)
Fig. 1. Modulation sensitivities of the red and green color mechanisms. (+) red and green mechanisms modulated equally, in phase (i.e. yellow luminance modulation). Filled symbols: modulation of only the red (0) and green (A) mechanisms, in the presence of an homogeneous yellow field. Open symbols: modulation of only the red (0) and green (A) mechanisms in the presence of an homogeneous 677 nm field.
the differences in sensitivity at 570 and 677 nm of Wald’s (1964) red and green “cone” functions.) The fact that the sensitivity of the red mechanism to high-frequency flicker is now similar to its sensitivity in the presence of the yellow field suggests that this is an appropriate correction. The sensitivity functions of the isolated red and green systems are similar, and both resemble the function that is measured with normal luminance modulation when both systems are stimulated. Thus, the low-frequency attenuation occurs within the individual mechanisms, and no additional attenuation is introduced when both are stimulated simultaneously. In the experiment that has just been described, stimuli that modulated the red and green systems were produced by two similar optical systems and combined at a half-silvered mirror. We could also present the two optical systems to separate eyes, in order to modulate the red mechanism of the right eye and the green mechanism of the left eye, and let the visual system combine the two inputs. No obvious rivalry takes place when viewing this stimulus, which is a 2” lighted area in a dark surround. The stimulus is indistinguishable in hue from a physical mixture of the same lights. The only phenomenal difference between the two situations is that the dichoptic mixture has a curious “metallic” appearance. When the red and green systems are modulated in counterphase, the appearance of the field is the same for either eye alone or both eyes together: it is a field that alternately becomes reddish and greenish. This demonstrates that the sensation of (e.g.) redness can result either from increased stimulation of the red mechanism or decreased stimulation of the green mechanism. The modulation sensitivity
for dichoptic
stimulation
(open
circles.
I
,,Sl,N,I
I(
/,\!I
1 Frequency
,
10
/,
I
I//l!
I
100
(Hz)
Fig. -1. Binocular and dichoptic modulation of red and green color mechanisms. (A): binocular stimulation of the red and green mechanisms of both eyes in phase (yellow luminance modulation). (A): binocular stimulation of the red and green mechanisms in counterphase. (0): dichoptic stimulation, in which the red mechanism of the right eye was modulated in counterphase to the green mechanism of the left eye. (The green mechanism of the right eye, and the red mechanism of the left eye. received a “compensated” stimulus, so that their quantum catch remained constant as the spectral composition of the stimulus varied.) (0): dichoptic stimulation, in which the red mechanism of the right eye and the green mechanism of the left eye were modulated in phase, while the green’ right and red/left were compensated.
Fig. 7) resembles that obtained with binocular stimulation, when the red and green systems of both eyes are modulated in counterphase. but the maximum sensitivity is slightly lower with dichoptic stimulation. When the red and green mehanisms are modulated in phase with dichoptic stimulation, the combined stimulus appears as a yellow field that is modulated in luminance, although if either eye is covered the field immediately appears to be modulated in hue. Again, the dichoptic thresholds (filled circles) are similar to the binocular thresholds even though each eye receives hue information that, if the other eye is covered, alfows the low-frequency sensitivity to rise to approximately the Ieve that is obtained with counterphase stimulation. Although hue information is present in the stimuli to the two eyes when both eyes are stimulated, the visual system is unable under these conditions to use this hue information to increase its sensitivity. Thus, detection of hue modulation must occur at a fairly high level in the visual system after convergence of information from the two eyes. The present results do not explain the observation by Estevez and Spekreijse that no low-frequency attenuation is found when sensitivity is measured with deep red stimuli that are modulated in luminance, which is a stimulus that would modulate only the red mechanism. In view of the present results, it seems that the detection of flicker in that situation must also be mediated by some cue other than brightness flicker; possibly b) some phenomenon that is similar to the Bezold-Briicke effect
3s I
Letrsr to rhe Editors .\n obwr\ation that suggests that the perception ot‘ ION-frequency tlicksr in deep red light is not a simple brightness discrimination is the following: after prolonged (10 min) adaptation to the red field. the lowfrequency sensitivity falls. so that the sensitivity function resembles that obtained w-ith white light at the same luminance level. Further. low-frequency attenuation is always found when the sensitivity of deuteranopic and protanopic observers is measured either ivith deep red light or with stimuli that modulate single color mechanisms in normal observers. This is consistent with. but does not prove, the notion that normal observers use hue information that is not available to the dichromats. Whatever the basis for the low-frequency sensitivity to red light. it now seems clear that the high-frequency branches of modulation sensitivity functions that are measured with the spectral compensation method retlect the sensitivity of single color mechanisms. However. at stimulus frequencies below 10 Hz, temporal discrimination of hue becomes important, and this hue information increases the measured sensititity. When hue modulation is removed by a superposed color. or by dichoptic stimulation, the sensitivity of the isolated red and green mechanisms has the same
low-frequency attenuation as the normal modulation sensitivity function that is measured with white light. It is unnecessary to postulate temporal interactions between the red and green mechanisms. Laborarorium coor Medischr Fysica, Hrrerlyracht 196, ,-lmsrrr&~m. Tlze Srrlwr~ufzds
0. ES-I&Z C. R. CAVOX~LJS
REFERESCES
De Lange H. (1958) Research into the dynamic nature of the human fovea-cortex systems with intermittent and modulated light. J. opr. Sot. .-lr~. 18, 779-739. Esttvez 0. and Spekreijse H. (19711 .A spectral compensation method for determining the flicker characteristics of human colour mechanisms. C’ision Rrs. 14, 523-530. Stiles W. S. (1953) Further studies of visual mechanisms by the two-colour method. In Co(oquio Sohre Problems Opricos de ia C’ision, pp. 65-103. Union Internationale de Physique pure et appliquee. Madrid. Wald G. (1964) The receptors of human color vision. Science. S.Y 145, 1007-1017. Wyszecki G. and Stiles W. S. (1967) Color Science. Wiley. New York.
. .
. .
l
l
I
0.1 100
,
0.1
,
/ , , , , /,(
,
,
/
/
, , , ,,
1
,
/
/
, , , , ,
j
100
10 Frequency I Hz)
Fig. 1. Modulation sensitivities of the red and green color mechanisms. (+) red and green mechanisms modulated equally, in phase (i.e. yellow luminance modulation). Filled symbols: modulation of only the red (0) and green (A) mechanisms, in the presence of an homogeneous yellow field. Open symbols: modulation of only the red (0) and green (A) mechanisms in the presence of an homogeneous 677 nm field.
the differences in sensitivity at 570 and 677 nm of Wald’s (1964) red and green “cone” functions.) The fact that the sensitivity of the red mechanism to high-frequency flicker is now similar to its sensitivity in the presence of the yellow field suggests that this is an appropriate correction. The sensitivity functions of the isolated red and green systems are similar, and both resemble the function that is measured with normal luminance modulation when both systems are stimulated. Thus, the low-frequency attenuation occurs within the individual mechanisms, and no additional attenuation is introduced when both are stimulated simultaneously. In the experiment that has just been described, stimuli that modulated the red and green systems were produced by two similar optical systems and combined at a half-silvered mirror. We could also present the two optical systems to separate eyes, in order to modulate the red mechanism of the right eye and the green mechanism of the left eye, and let the visual system combine the two inputs. No obvious rivalry takes place when viewing this stimulus, which is a 2” lighted area in a dark surround. The stimulus is indistinguishable in hue from a physical mixture of the same lights. The only phenomenal difference between the two situations is that the dichoptic mixture has a curious “metallic” appearance. When the red and green systems are modulated in counterphase, the appearance of the field is the same for either eye alone or both eyes together: it is a field that alternately becomes reddish and greenish. This demonstrates that the sensation of (e.g.) redness can result either from increased stimulation of the red mechanism or decreased stimulation of the green mechanism. The modulation sensitivity
for dichoptic
stimulation
(open
circles.
I
,,Sl,N,I
I(
/,\!I
1 Frequency
,
10
/,
I
I//l!
I
100
(Hz)
Fig. -1. Binocular and dichoptic modulation of red and green color mechanisms. (A): binocular stimulation of the red and green mechanisms of both eyes in phase (yellow luminance modulation). (A): binocular stimulation of the red and green mechanisms in counterphase. (0): dichoptic stimulation, in which the red mechanism of the right eye was modulated in counterphase to the green mechanism of the left eye. (The green mechanism of the right eye, and the red mechanism of the left eye. received a “compensated” stimulus, so that their quantum catch remained constant as the spectral composition of the stimulus varied.) (0): dichoptic stimulation, in which the red mechanism of the right eye and the green mechanism of the left eye were modulated in phase, while the green’ right and red/left were compensated.
Fig. 7) resembles that obtained with binocular stimulation, when the red and green systems of both eyes are modulated in counterphase. but the maximum sensitivity is slightly lower with dichoptic stimulation. When the red and green mehanisms are modulated in phase with dichoptic stimulation, the combined stimulus appears as a yellow field that is modulated in luminance, although if either eye is covered the field immediately appears to be modulated in hue. Again, the dichoptic thresholds (filled circles) are similar to the binocular thresholds even though each eye receives hue information that, if the other eye is covered, alfows the low-frequency sensitivity to rise to approximately the Ieve that is obtained with counterphase stimulation. Although hue information is present in the stimuli to the two eyes when both eyes are stimulated, the visual system is unable under these conditions to use this hue information to increase its sensitivity. Thus, detection of hue modulation must occur at a fairly high level in the visual system after convergence of information from the two eyes. The present results do not explain the observation by Estevez and Spekreijse that no low-frequency attenuation is found when sensitivity is measured with deep red stimuli that are modulated in luminance, which is a stimulus that would modulate only the red mechanism. In view of the present results, it seems that the detection of flicker in that situation must also be mediated by some cue other than brightness flicker; possibly b) some phenomenon that is similar to the Bezold-Briicke effect
3s I
Letrsr to rhe Editors .\n obwr\ation that suggests that the perception ot‘ ION-frequency tlicksr in deep red light is not a simple brightness discrimination is the following: after prolonged (10 min) adaptation to the red field. the lowfrequency sensitivity falls. so that the sensitivity function resembles that obtained w-ith white light at the same luminance level. Further. low-frequency attenuation is always found when the sensitivity of deuteranopic and protanopic observers is measured either ivith deep red light or with stimuli that modulate single color mechanisms in normal observers. This is consistent with. but does not prove, the notion that normal observers use hue information that is not available to the dichromats. Whatever the basis for the low-frequency sensitivity to red light. it now seems clear that the high-frequency branches of modulation sensitivity functions that are measured with the spectral compensation method retlect the sensitivity of single color mechanisms. However. at stimulus frequencies below 10 Hz, temporal discrimination of hue becomes important, and this hue information increases the measured sensititity. When hue modulation is removed by a superposed color. or by dichoptic stimulation, the sensitivity of the isolated red and green mechanisms has the same
low-frequency attenuation as the normal modulation sensitivity function that is measured with white light. It is unnecessary to postulate temporal interactions between the red and green mechanisms. Laborarorium coor Medischr Fysica, Hrrerlyracht 196, ,-lmsrrr&~m. Tlze Srrlwr~ufzds
0. ES-I&Z C. R. CAVOX~LJS
REFERESCES
De Lange H. (1958) Research into the dynamic nature of the human fovea-cortex systems with intermittent and modulated light. J. opr. Sot. .-lr~. 18, 779-739. Esttvez 0. and Spekreijse H. (19711 .A spectral compensation method for determining the flicker characteristics of human colour mechanisms. C’ision Rrs. 14, 523-530. Stiles W. S. (1953) Further studies of visual mechanisms by the two-colour method. In Co(oquio Sohre Problems Opricos de ia C’ision, pp. 65-103. Union Internationale de Physique pure et appliquee. Madrid. Wald G. (1964) The receptors of human color vision. Science. S.Y 145, 1007-1017. Wyszecki G. and Stiles W. S. (1967) Color Science. Wiley. New York.
